This invention relates to the art of nuclear reactors and it has particular relationship to burnable neutron-absorber assemblies, also called burnable-poison assemblies, for nuclear reactors. The burnable neutron-absorber assemblies with which this invention concerns itself are of the type described in Radford application. Such neutron-absorber assemblies include annular ceramic pellets which are stacked in tubes inserted in the core of a reactor. It is with the pellets that this invention concerns itself. Such a pellet includes a matrix of a refractory material which may include aluminum oxide (Al.sub.2 O.sub.3) or zirconium oxide ZrO.sub.2 or a combination of the two. A neutron absorber or neutron-capture component is distributed throughout this matrix. The neutron absorber may include one or more elements or compounds of the metals boron, gadolinium, samarium, cadmium, europium, hafnium, dysprosium and indium. A neutron absorber commonly used is boron carbide (B.sub.4 C) either natural or with the boron enriched B.sup.10.
In the interest of brevity and concreteness to facilitate the understanding of those skilled in the art in the practice of this invention, this application will deal specifically with a matrix of Al.sub.2 O.sub.3 and a neutron absorber of B.sub.4 C. It is understood that to the extent that this invention is practiced with other materials, such practice is within the scope of equivalents of this invention as scope of equivalents is defined and described in the Supreme Court Grover case cited in Radford application.
The method of producing pellets disclosed in Radford application and the pellets produced thereby have proven themselves highly satisfactory. However, experience with this method and the pellets produced thereby had led to the conclusion that several improvements are desirable. It is desirable that the pores or voids in the matrix be more efficiently or effectively used to take up the expansion of the B.sub.4 C and absorb the helium gas generated by the neutron-boron reaction. It is also desirable that the strength, particularly the compressive strength, of the matrix be improved. It is an object of this invention to provide a method for producing neutron-absorbing bodies or ceramics having the above-described desirable properties. It is also an object of this invention to provide a neutron-absorbing body or ceramic having the above desirable properties.
In this application the expression "neutron-absorber assembly" or "poison assembly" means the neutron-absorber structure or rod as a whole including the pellets and the container in which the pellets are stacked; "neutron absorber" means the neutron-capture component, e.g., B.sub.4 C; "neutron-absorber body" means the body including the neutron absorber in its matrix.